The present invention generally concerns three-dimensional optical memory apparatus and memory media, and methods of using such apparatus and media. The present invention particularly concerns (i) three-dimensional volumes of an active, radiation-sensitive, medium that is selectively both alterable and interrogatable by use of at least two intersecting beams of radiation, thereby to form a radiation memory; (ii) the manner of using the intersecting radiation beams and the physical and/or chemical effects of such use; (iii) the construction of binary-stated informational memory stores, three-dimensional patterns, and/or three-dimensional displays based on these effects; (iv) the manner of selectively directing radiation beams to intersect within three-dimensional volumes for purposes of addressing selected domains within such volumes, particularly to serve as an addressable memory store; and (v) the manner of selectively impressing information on, or extracting information from, one or more intersecting beams of radiation in order that such information may be radiatively written to, or radiatively read from, a three-dimensional volume memory.
The need for computerized data storage and processing has been increasing, in the past decade, at a high rate. In response to this need, semiconductor-based computer technology and architecture have greatly improved. However, barriers to further reducing the size and price of semiconductors may now be inhibiting development of even higher performance computers, and the more widespread use of high performance computers.
The major determinant of the size and price of high performance computers is the memory. The data storage requirements of new high performance computers, circa 1994, are very great, typically many gigabytes (1012 bits). New and improved, compact, low cost, very high capacity memory devices are needed. These memory devices should be able to store many, many gigabytes of information, and would desirably randomly retrieve such information at the very fast random access speeds demanded by parallel computing.
An optical memory offers the possibility of packing binary-stated information into a storage medium at very high density, with each binary bit occupying a space only about one light wavelength in diameter. When practical limitations are taken into account this leads to a total capacity of about 1011 bits for a reasonably-sized two-dimensional optical storage mediumxe2x80x94the amount of information contained in about 3000 normal size books. A comparison of the optical memory to existing types of computer memories is contained in the following Table 1.
The present invention will be seen to be embodied in an optical memory system. Any optical memory system, whether three-dimensional (3-D), four-dimensional (4-D), or otherwise, is based on light-induced changes in the optical, chemical and/or physical properties of materials.
At the present two general types of optical recording media exist, namely phase recording media and amplitude recording media. Recording on the media of the first type is based on light-induced changes of the index of refraction (i.e., phase holograms). Recording on the media of the second type is based on photo-induced changes in the absorption coefficient (i.e., hole burning).
Volume information storage is a particularly attractive concept. In a two dimensional memory the theoretical storage density (proportional to 1/wavelength xcex2) is 1xc3x971011 bits/cm2 for xcex=266 nm. However in a 3-D memory the theoretical storage density is 5xc3x971016 bits/cm3. Thus the advantages of 3-D data storage versus previous two dimensional information storage media become apparent.
Volume information storage has previously been implemented by holographic recording in phase recording media. Reference F. S. Chen, J. T. LaMacchia and D. B. Fraser, Appl. Phys. Lett., 13, 223 (1968); T. K. Gaylord, Optical Spectra, 6, 25 (1972); and L. d""Auria, J. P. Huignard, C. Slezak and E. Spitz, Appl. Opt., 13, 808 (1974).
The present invention will be seen to implement volume writable-readable-erasable optical storage in a phase recording medium that is also, coincidentally, an amplitude recording medium. One early patent dealing with three-dimensional amplitude-recording optical storage is U.S. Ser. No. 3,508,208 for an OPTICAL ORGANIC MEMORY DEVICE to Duguay and Rentzepis, said Rentzepis being the selfsame inventor of the present invention. Duguay and Rentzepis disclose an optical memory device including a two-photon fluorescent medium which has been solidified (e.g., frozen or dispersed in a stable matrix, normally a polymer). Information is written into a selected region of the medium when a pair of picosecond pulses are made to be both (i) temporally coincident and (ii) spatially overlapping within the selected region. The temporally-coincident spatially-overlapping pulses create, by process of two-photon absorption, organic free radicals which store the information at an energy level intermediate between a fluorescent energy level and a ground state energy level. The free radicals store the desired information for but a short time, and until they recombine. The information may be read out by interrogating the medium with a second pair of coincident and overlapping picosecond pulses. In the case where the medium is frozen solid, interrogation may also be accomplished by directing a collimated infrared light beam into the selected region, thereby causing that region to liquefy and permitting its contained free radicals to undergo recombination. In each of the aforementioned cases, the interrogation beam causes the interrogated region to selectively fluoresce in accordance with the presence, or absence, or free radicals. The emitted radiation is sensed by an appropriate light detector as an indication of the informational contents of the interrogated region.
This early optical memory of Duguay and Rentzepis recognizes only that two-photon absorption should be used to produce excited states (e.g., singlet, doublet or triplet states) of an radiation-sensitive medium over the ground state of such medium. These excited states are metastable. For example, one preferred fluorescent medium is excitable from ground to a singlet state by process of two-photon absorption occurring in about 10xe2x88x9215 second. The excited medium will remain in the singlet state for about 10xe2x88x928 second before fluorescing and assuming a metastable triplet state. This metastable state represents information storage. Alas, this metastable state will spontaneously decay to the ground state by fluorescence after about 1 second (depending on temperature). The memory is thus unstable to hold information for periods longer than about 1 second. It should be understood that the fluorescent medium of the Duguay and Rentzepis memory is at all times the identical molecular material, and simply assumes various excited energy states in response to irradiation.
Another previous optical system for accomplishing the volume storage of information, and for other purposes, is described in the related series of U.S. Pat. Nos. 4,078,229; 4,288,861; 4,333,165; 4,466,080; and 4,471,470 to Swainson, et al. and assigned to Formigraphic Engine Corporation. The Swainson, et al. patents are variously concerned with three-dimensional systems and media for optically producing three-dimensional inhomogeneity patterns. The optically-produced 3-D inhomogeneity patterns may exhibit (i) controlled refractive index distributions, (ii) complex patterns and shapes, or (iii) physio-chemical inhomogeneities for storing data. The Swainson, et al. patents generally show that some sort of chemical reaction between two or more reactive components may be radiatively induced at selected cell sites of a 3-D medium in order to produce a somewhat stable, changed, state at these selected sites.
U.S. Pat. No. 4,471,470, in particular, describes a METHOD AND MEDIA FOR ACCESSING DATA IN THREE-DIMENSIONS. Two intersecting beams of radiation are each matched to a selected optical property or properties of the active media. In one embodiment of the method and media, called by Swainson, et al. xe2x80x9cClass I systems,xe2x80x9d two radiation beams generate an active region in the medium by simultaneous illumination. In order to do so, two different light-reactive chemical components are typically incorporated within the medium. Both components are radiation sensitive, but to different spectral regions. The two radiation beams intersecting in a selected region each produce, in parallel, an associated chemical product. When two products are simultaneously present in a selected intersection region then these products chemically react to form a desired sensible object. The sensible object may represent a binary bit of information. One or both of the radiation-induced chemical products desirably undergoes a rapid reverse reaction upon appropriate irradiation in order to avoid interference effects, and in order to permit the three-dimensional media to be repetitively stored.
In other embodiment of the Swainson et al. method and media, called xe2x80x9cClass II systems,xe2x80x9d one of the radiation beams must act on a component of the medium before the medium will thereafter be responsive to the other radiation beam. The class I and class II systems thus differ by being respectively responsive to the effects of simultaneously, and sequentially, induced photoreactions.
The Swainson, et al. patentsxe2x80x94including those patents that are not directed to information storage and that are alternatively directed to making optical elements exhibiting inhomogeneity in their refractive index, or to making physical shapes and patternsxe2x80x94are directed to inducing changes in a bulk media by impinging directed beams of electromagnetic radiation, typically laser light, in order that selected sites within the bulk media may undergo a chemical reaction. There are a large number of photosensitive substances that are known to undergo changes in the presence of light radiation. The changed states of these substances are, in many cases, chemically reactive. The patents of Swainson, et al. describe a great number of these photosensitive and photo-reactive substances. Such substances may generally be identified from a search of the literature.
Swainson, et al. also recognize that molecular excitation from a ground state to an excited state may occur following a stepwise absorption of two photons. Swainson, et al. call this xe2x80x9ctwo-photon absorption.xe2x80x9d Swainson, et al. describe that a solution of 8xe2x80x2allyl-6xe2x80x2nitro-1,3,3-trimethylspiro(2xe2x80x2N-1-benzopyran-2xe2x80x2-2-induline) in benzene may be exposed to intersecting synchronized pulsed ruby laser beams transmitted through an UV elimination filter to form, at the region of intersection, a spot of color. The process of stepwise absorption of two photons in this solution, and in others, is recognized by Swainson, et al., only as regards its use to produce an excited state that may form (as in the example) colored products, or that may serve as an energy transfer agent.
In making all manner of excited statesxe2x80x94including singlet, doublet, triplet, and quartet statesxe2x80x94the patents of Swainson, et al. describe known photochemistry. Generally chemistry, and photochemistry, that is known to work in one dimension is equally applicable in three dimensions. For example, it is known that an electron may be knocked off an active substance so that it becomes an ion. For example, it is known that radiation may cause a substance to dissociate a proton, again becoming an ion. For example, it is even known how to induce spin changes and changes in parity by electromagnetic radiation. Once these changes, or others, are induced in an radiation-sensitive medium then Swainson, et al. describe a reliance on the transport capabilities of the liquid or gaseous support media in order to permit a chemical reaction to transpire.
The present invention will be seen to reject the Swainson et al. approach of inducing chemical reactions in a 3-D medium by creating one or more reagents by use of radiation. One reason why the present invention does so is because the same support medium, or matrix, that offers those transport capabilities that are absolutely necessary to permit the chemical reactions to occur will also permit, at least over time, undesired migration of reagents or reaction products in three dimensions, destroying the integrity of the inhomogeneity pattern.
The inventions of both related patent applications contemplate (i) addressing, and (ii) writing data to or reading data from, selected domains within a three-dimensional volume of radiation-sensitive medium by and with two selectively chosen, coincident, radiation beams. The radiation beams are selectively guided to spatial and temporal coincidence so as to cause certain selected domains, and only those certain selected domains, to selectively undergo selected changes by process of two-photon absorption.
The first co-pending patent applicationxe2x80x94Ser. No. 342,978 filed Apr. 25, 1989 issued Dec. 7, 1993 as U.S. Pat. No. 5,268,862 for a THREE-DIMENSIONAL OPTICAL MEMORYxe2x80x94particularly teaches selectively inducing isomeric changes in the molecular isomeric form of selected regions within a three-dimensional radiation-sensitive medium by the process of two-photon absorption.
The method of the related invention produces a three-dimensional inhomogeneity pattern in a volume of active media in response to directed electromagnetic radiation. In order to do so, an radiation-sensitive medium having at least two isomeric molecular forms is contained within a volume. The radiation-sensitive medium is responsive to energy level changes stimulated by electromagnetic energy to change from one of its isomeric molecular forms to another of its isomeric molecular forms. A selected portion of the radiation-sensitive medium is selectively radiated with plural directed beams of electromagnetic radiation to change the selected portion from the one isomeric molecular form to the other isomeric molecular form by process of plural-photon absorption. The induced isomeric changes possess useful optical, chemical, and/or physical characteristics.
In the preferred embodiment of the first predecessor invention an radiation-sensitive medium, typically a photochromic material and more typically spirobenzopyran, was maintained in a three-dimensional matrix, typically of polymer, and illuminated in selected regions by two UV laser light beams, typically of 532 nm. and 1064 nm. wavelength. The illumination cause the radiation-sensitive, photochromic, spirobenzopyran medium to change from a first, spiropyran, to a second, merocyanine, stable molecular isomeric form by process of two-photon absorption. Regions not temporally and spatially simultaneously illuminated were unchanged. Later illumination of the selected regions by two green-red laser light beams, typically of 1064 nm wavelength each, caused only the second, merocyanine, isomeric form to fluoresce. This fluorescence was detectable by photodetectors as stored binary data. The three-dimensional memory can be erased by heat, or by infrared radiation, typically 2.12 microns wavelength. Use of other medium permit the three-dimensional patterning of three-dimensional forms, such as polystyrene polymer solids patterned from liquid styrene monomer. Three-dimensional displays, or other inhomogeneity patterns, can also be created.
The present application will be seen to use, as one suitable radiation-sensitive, photochromic, medium the exact same medium as did the volume optical memory of the predecessor invention: spirobenzopyran. However, the present invention will be seen to make use of a property other than fluorescence that also varies between the two sable isomeric molecular forms of the spirobenzopyran molecule. This property is the index of refraction.
Meanwhile, the second co-pending patent applicationxe2x80x94Ser. No. 586,456 filed Sep. 21, 1990 for a THREE-DIMENSIONAL OPTICAL MEMORYxe2x80x94particularly deals with a system and method for addressing a three-dimensional radiation memory with two radiation beams so as to, at separate times, write binary data to, and to read binary, data from, such memory by process of two-photon absorption. The radiation beams are typically, but not necessarily, light, and are more typically laser light. Accordingly, the complete device, or system, incorporating such a volume memory was called a two-photon 3-D optical memory, or a 2-P 3-D OM.
The addressing of the volume memory within the 2-P 3-D OM preferably (but not necessarily) used, as a part of one component (a holographic dynamic focusing lens, or HDFL), a hologram. Thus the 2-P 3-D OM was preferably holographically addressed.
The optical memory of the present invention will be seen to dispense with the requirement for a HDFL, or for holographic addressing.
In the 2-P 3-D OM of the second predecessor invention one directed beam of electromagnetic radiation was spatially encoded as an nxc3x97n wavefront array of binary bits by use of a two-dimensional spatial light modulator (2-D SLM). This spatially-encoded beam, and an additional, orthogonal, beam of electromagnetic radiation, were then selectively guided into spatial and temporal coincidence at a selected nxc3x97n planar array of domains within a three-dimensional matrix of such domains within a three-dimensional volume of radiation-sensitive medium.
This function of the optical memory of the second predecessor invention to spatially encode information upon a planar wavefront, or pulse, of radiation will be seen to be continued in the optical memory of the present invention.
In the second predecessor invention, the spatially-encoded selectively-guided coincident radiation beams served, dependent upon their combined energies, to either write (change) or read (interrogate) the condition, and particularly the isomeric molecular form, of the selected domains by a process of two-photon absorption. Remaining, unselected, domains received insufficient (i) intensity from either beam, or (ii) combined energy from both beams, so as to be substantially affected.
This function, and property, will also be seen to be preserved in the optical memory of the present invention.
In its preferred embodiment, the optical memory of the second predecessor invention served to temporally and spatially simultaneously illuminate by two radiation beamsxe2x80x94normally laser light beams in various combinations of wavelengths 532 nm and 1024 nmxe2x80x94certain selected domainsxe2x80x94normally 103xc3x97103 such domains arrayed in a planexe2x80x94within a three-dimensional (3-D) volume of radiation-sensitive mediumxe2x80x94typically 1 cm3 of spirobenzopyran containing 102 such planes. The selective illumination served, dependent upon the particular combination of illuminating light, to either write binary data to, or read binary data from, the selected domains by process of two-photon (2-P) absorption. One laser light beam was preferably directed to illuminate all domains of the selected plane in and by a one-dimensional spatial light modular (1-D SLM). The other laser light beam was first spatially encoded with binary information by 2-D SLM, and was then also directed to illuminate the domains of the selected plane. Direction of the binary-amplitude-encoded spatially-encoded light beam was preferably by focusing, preferably in and by a holographic dynamic focusing lens (HDFL). During writing the selected, simultaneously illuminated, domains changed in their isomeric molecular form by process of 2-P absorption. During reading the selected domains fluoresced dependent upon their individually pre-established, written, states. The domains"" fluorescence was focused by the HDFL, and by other optical elements including a polarizer and polarizing beam splitter, to a 103xc3x97103 detector array. The I/O bandwidth to each cm3 of radiation-sensitive medium was on the order of 1 Gbit/sec to 1 Tbit/sec.
The three-dimensional optical memory of the present invention will be seen to employ spatial light modulators, as did the optical memories of the related predecessor inventions.
A recent survey, circa 1990, of spatial light modulators is contained in the article Two-Dimensional Spatial Light Modulators: A Tutorial by John A. Neff, Ravindra A. Athale, and Sing H. Lee, appearing in Proceedings of the IEEE Vol. 78, No. 5, May 1990 at page 826. The following summary is substantially derived from that article.
Two-dimensional Spatial Light Modulators (SLMs) are devices that can modulate the properties of an optical wavefrontxe2x80x94such as the properties of amplitude, phase, or polarizationxe2x80x94as a function of (i) two spatial dimensions and (ii) time in response to information-bearing control signals that are either optical or electrical. SLMs usefully form a critical part of optical information processing systems by serving as input transducers as well as performing several basic processing operations on optical wave fronts.
SLMs, although once considered simply as transducers that permitted the input of information to an optical processor, have a broad range of applications, and are capable of performing a number of useful operations on signals in the optical domain. Some of the more important functions that have been demonstrated with SLMs are: analog multiplication and addition, signal conversion (power amplification, wavelength, serial-to-parallel, incoherent-to-incoherent, electrical-to-optical), nonlinear operations, and short-term storage.
The functional capabilities of SLMs can be exploited in a wide variety of optical computer architectures. Applications of 1-D and 2-D SLMs encompass just about every optical signal processing/ computing architecture conceived.
SLMs may be classified as to type. The major classification categories result from (i) the optical modulation mechanism, (ii) the variable of the optical beam that is modulated, (iii) the addressing mode (electrical or optical), (iv) the detection mechanism (for optically-addressed SLMs), and (v) the addressing mechanism (for electrically-addressed SLMs).
The modulation of at least one property of a readout light beam is inherent in the definition of an SLM. Hence the first major category of SLMs is based on modulation mechanisms. The modulation mechanism employs an intermediate representation of information within a modulating material. An information-bearing signal, either optical or electrical, is converted into this intermediate form. The major forms of conversion mechanisms that are employed in 2-D SLMs are
(a) Mechanical
(b) Magnetic
(c) Electrical
(d) Thermal.
Of these conversion mechanisms, the electrical mechanism will be seen to be preferred for use in the three-dimensional optical memory of the present invention. In the electrical conversion mechanism, the electric field interacts with the modulating material at several levels, giving rise to different effects. The interaction can take the form of distorting the crystal lattice, changing the molecular orientations, or modulating the electron density functions.
A conversion mechanism and the modulating material so converted have a characteristic response time, activation energy, and spatial scale. These parameters, in turn, have a major impact on the respective speed sensitivity and spatial resolution of the optical modulation performed by the SLM. A modulation mechanism, however, becomes physically more specific only when combined with a choice of appropriate modulation variables, to be discussed next.
An optical wavefront has several associated variables that can be modulated as a function of the spatial coordinates and time in order to carry information. These variables include
(a) Intensity (amplitude)
(b) Phase
(c) Polarization
(d) Spatial frequency spectrum (texture).
Intensity (amplitude) and phase are the most commonly used representations in an optical computing system. Polarization and spatial frequency spectrum are often used as intermediate representations, and are converted into intensity or phase modulation before the information is used in the next stage of the optical computing system. Intensity (amplitude), phase, and polarization modulation will each be seen to be employed in the three-dimensional optical memory of the present invention.
Intensity, or amplitude, modulation commonly results when the absorption characteristics of a modulating material are changed. Because the intensity of a light beam is proportional to the square of its amplitude, the difference between these two modes depends on the variable that is employed in subsequent processing of a SLM output. The present invention will be seen to be more concerned with selectively controllably spatially modulating to zero intensity, and amplitude, then with any requirement that modulation at and to an opposite binary state should produce sufficient intensity, and amplitude, so as to permit a desired operation within an optical memory. This is because any presence of light intensity, or amplitude, in those spatial locations of an optical wavefront (i.e., at a particular time) where, and when, there is desirably no light intensity, nor any amplitude, constitutes optical noise.
The three-dimensional optical memory in accordance with the present invention will be seen to be innately highly insensitive to optical noise, being roughly sensitive to (noise/signal)2, as opposed to the lesser figure of merit noise/signal, in certain operations. Nonetheless to this innate insensitivity, optical noise may be cumulative in degrading the integrity of informational stores within the optical memory over billions and trillions of read and write cycles. Accordingly, intensity, or amplitude, modulation in accordance with the present invention is desirably very xe2x80x9cclean,xe2x80x9d with minimal, essentially zero, optical intensity or amplitude in those wavefront regions which are spatially modulated to one (xe2x80x9c0xe2x80x9d) binary state. Spatial light modulation, and SLMs, will be seen to so operate in the present invention: veritably no light will be in regions where it is not wanted.
Polarization modulation is commonly achieved by modulating the birefringence associated with the modulating material of the SLM. Birefringence is a property of some materials in which the refractive index depends on the state of polarization and direction of light propagation. Depending upon the effect utilized, the state of polarization changes (e.g., from linear to elliptical), or the angle of the linear polarization changes without changing the state of polarization. The memory system of the present invention will be seen to use phase-modulating SLMs that produce each such effect.
Polarization modulation can be changed into intensity (amplitude) modulation by employing polarized readout light and an analyzer in the output. The memory system of the present invention will later be seen to be so change polarization modulation into intensity modulation. Indeed, this will be seen to be a primary approach by which the net effective intensity, or amplitude, modulation will be rendered exceptionally xe2x80x9cclean,xe2x80x9d and of satisfactory quality to support reliable operation of the three-dimensional optical memory over great periods of time and astronomical numbers of read and write cycles.
Most new memory technologies are typically immediately gauged by the figures of merit that have attended past technologies. These previous figures of merit, while generally representing criteria that must be met by an operational memory, are often substantially irrelevant to the truly critical performance aspects, and new figures of merit, appropriate to a new technology.
For example, the Intellectual Property Owners, Inc. gives annual awards in the name of its educational subsidiary the IPO Foundation to distinguished inventors. In the 1989 awards, Robert P. Freese, Richard N. Gardner, Leslie H. Johnson and Thomas A. Rinehart were honored for their improvements in erasable, re-writable optical disks introduced by the 3M Company during 1988. The optical disks can store 1,000 times as much information as conventional flexible diskettes used with personal computers. The inventors were the first to achieve a signal to noise ratio for an erasable optical disk in excess of 50 decibels.
Although the inventors of the present invention would be the first to recognize this contribution, and to acknowledge the necessity of an adequate optical (and electrical) signal-to-noise ratio for optical memories, a focus on signal-to-noise as a figure of merit may be rooted in the importance of this measurement in certain previous electrotechnology. For example, certain magnetic memories, such as garnet film and Block line memories, have undesirably small signal-to-noise ratios.
It is uncertain what constitutes the ultimate, or even the most appropriate, figure of merit (or figures of merit) for a readable and writable and erasable optical memory. However, it is suggested that, in the case of a three-dimensional optical memory, it is important to consider whether or not, and how fast, the memory might become xe2x80x9cdirtyxe2x80x9d from use and suffer degradation in the integrity of its data stores.
The concept of a xe2x80x9cdirtyxe2x80x9d three-dimensional optical memory arises because every read and write operation on the memory by use of radiation has the potential to perturb other storage domains than just those domains that are intended to be dealt with. The most analogous prior memory technology may be the original square loop ferrite magnetic core memories. In these early core memories many millions of interrogations of one memory location may cause a single magnetized core having a weak hysteresis to fail to provide a sense signal adequate for detection of its magnetic condition, meaning the binary data bit stored. Even more relevantly, unaddressed and/or unwritten cores, commonly in physically proximate positions, may sometimes inadvertently and erroneously change hysteresis state, causing attendant loss of data.
A three-dimensional optical memory is analogous. The radiation that is used to read and write selective domains of the memory can, if great care is not employed, end up, after millions or billions of cumulative cycles, changing domains other than those domains that are desired to be changed. Such an undesired change of domains degrades the integrity of the data stored within the memory.
Accordingly, the present invention concerns not only addressably reading and writing and erasing a three-dimensional optical memory and doing so at impressive levels of performance, but doing so by design, at a high figure of merit. A xe2x80x9chigh figure of meritxe2x80x9d means that an optical memory constructed in accordance with the invention is practically and reliably useful in the real world, reliably storing and reading any and all data patterns with absolute integrity during indefinitely long periods of any pattern of use, or non-use, whatsoever. Consider that three-dimensional optical memories, storing information in a volume that is little more than a cube of plastic, are intrinsically physically amorphous and homogenous. It is prudent to use some care, and forethought, in the manner of radiative reading and writing of such a volume so that those changes that are selectively induced within selected domains of the volume should be absolutely stable and independent. Nothing should be done, or repetitively done, on any selected domains that adversely affects the integrity of non-selected domains.
The present invention contemplates a random-access non-destructive-readout xe2x80x9cfour dimensionalxe2x80x9d (xe2x80x9c4-Dxe2x80x9d) radiation memory that is selectively repetitively addressably writable and non-destructively readable (and, less commonly, erasable) by process of two-photon (xe2x80x9c2-Pxe2x80x9d) interaction.
In various preferred embodiments of the invention a three-dimensional volume of radiation-sensitive medium, typically a photochromic material and more typically spirobenzopyran, maintained in a matrix, typically a 1 cm3 cubical polymer matrix, is selectively written by two selectively controllably time-phased UV laser light pulses, typically two 10 picosecond pulses of 532 nm and 1064 nm wavelength. One of the radiation pulses is two-dimensionally spatially encoded in its wavefront. The two radiation pulses are (i) spatially directed, including so as to counter-propagate at 180xc2x0 opposition to each other, and (ii) temporally phase-sequenced, so as to temporally and spatially intersect, satisfying the quantum mechanical equations of two-photon interaction, within only a portion of the volume, typically a plane of (2xc3x97103)2 arrayed domains.
During writing, both illuminating write radiation pulses pass substantially unobstructed straight through the volume of the radiation-sensitive medium, temporally and spatially simultaneously intersecting inside this volume substantially only in a defined locus of intersection domains. The index of refraction of these intersection domains is selectively changed by process of two-photon absorption. Other domains not temporally and spatially simultaneously illuminated are totally unchanged.
During subsequent reading, two green-red laser light beams, typically of 1064 nm wavelength each, pass substantially unobstructed through the same volume, each interacting with the index (indices) of refraction thereof substantially only by two-photon interaction and substantially only in a locus of intersection domains. The interaction of each green-red laser light beam in, and substantially only in, the precisely-defined intersection domains is detectable by redundant arrayed photodetectors as binary data stored in the intersection domains.
During reading with the two green-red laser light beams the medium at each intersection domain also selectively fluoresces dependent upon its pre-existing index of refractionxe2x80x94which index of refraction is, in the preferred spirobenzopyran radiation-sensitive medium, also indicative of the isomeric molecular form of the medium. This fluorescence, occurring at a separate wavelength and frequency from each of the selectively transmitted read light pulses, is also detectable as the binary data stored in the intersection domains. Each frequency of light output being a separate dimension, the two-photon (xe2x80x9c2-Pxe2x80x9d) radiation memory is four dimensional, or xe2x80x9c4-Pxe2x80x9d.
A 2-P 4-D radiation memory in accordance with the present invention stores binary information in a three-dimensional (3-D) volume of a medium that is sensitive to radiation in its absorption band so as to undergo an anomalous, stable, change in its index of refraction. For simplicity, this medium is called a xe2x80x9cradiation-sensitive mediumxe2x80x9d. The 3-D volume may be, for example, in the shape of a cube of typical size 1 cm3. The cube may typically contain as a radiation-sensitive medium the photochromic chemical spirobenzopyran stably held in position in and by a matrix of, for example, polymer plastic.
The cubical 3-D volume of radiation sensitive medium is, accordance with the present invention, simultaneously momentarily radiatively illuminated along each of two axis that are mutually intersecting at a predetermined angle. The illumination axis are typically intersecting either at 90,xe2x80x94perpendicularlyxe2x80x94or at 180,xe2x80x94in which case the two axis are really but one axis along which the two radiation illuminations are counter-propagating.
The illuminating radiation along at least one, and preferably both, of the intersecting axis is in the form of momentary pulse. The pulse can be in the form of a plane wave, alternatively called a planar wave front. At least one illuminating radiation pulse is commonly in this form, and both pulses are preferrably in this form and must be in this form particularly in the counter-propagating intersection geometry.
The momentary, pulse, illumination serves to define and to addressably select, in a manner to be explained, a unique multiplicity of domainsxe2x80x94for example 4xc3x97104 such domainsxe2x80x94out of a very great multiplicity of such domainsxe2x80x94for example out of 8xc3x97109 such domains that are three-dimensionally (3-D) arrayed within the volume. Each selected multiplicity of domains is substantially two-dimensional, meaning that it is but one domain xe2x80x9cthickxe2x80x9d in the direction of each, and of both, illuminations. The selected multiplicity of domains can be, but need not be, in a plane (irrespective of whether either or both illuminating radiation pulses in a planar wavefront). Note that the domains are defined by the illumination, and do not represent initially, or permanently, physically or otherwise-differentiated regions within the volume of radiation-sensitive medium, which is substantially homogeneous.
Each domain stores binary information as one of two stable states, each of which states has a different associated index of refraction.
Domains within the 3-D volume of radiation-sensitive medium are written (changed from a first to a second state and associated index of refraction) by process of two-photon absorption. Domains are so written upon such times as two time-resolved radiation beams, or radiation pulses, together having a joint energy that aggregates a predetermined first energy level both (i) temporally and (ii) spatially intersect within the domain. (Remember always that the energy of a radiation beam, or radiation pulse, is a function of its frequency (E=hxcexd), and not of either its intensity or its duration.) The radiation-sensitive medium within each intersection domain is responsive to radiation of this first energy level to change from a first one of its two stable states to its other, second, state.
The responsiveness of the radiation-medium to so change is a function of the well-known quantum-mechanical equations of two-photon interaction, particularly (in the case of writing the medium) two-photon absorption. The necessity, in accordance with the law of physics, of having a temporal, as well as a spatial overlap between the two intersecting radiation pulses will prove important to the present invention. Namely, in accordance with the present invention the (i) durations and (ii) time sequence (relative phase) of the illuminating pulses, as well as their spatial directions, will be positively controlled. The size and locations of the domains, as well as the nature of changes in the state and the percentage completeness of such changes within the volume of the domain, is a function of the time (i) duration and (ii) sequence of the illuminating radiation pulses, as well as the direction of such pulses. Indeed, the directions, or axis, of both illuminating radiation pulses are normally maintained constant, for example at 90xc2x0 or 180xc2x0, for a particular embodiment of the invention. Moreover, there is no selective focusing, nor any other attempt to manipulate the spatial direction or density distribution of the illuminating radiation pulses. Each pulse typically impinges as a plane wave parallel to a face of a cubical volume straight against a face of the cube, and passes straight through the entire volume. Pulse timing is the all-important determinate of where changes (in the event of writing or erasing), or detections (in the event of reading), transpire within the volume of radiation-sensitive medium.
This is considerably different than most, if not all, previous optical volume memory addressing schemes where radiation beams were simply directionally concentrated upon the volume portions where changes were to be made (possibly even by process of two-photon absorption), but where the illuminating beams were not positively temporally (as well as spatially) controlled. In accordance with the present invention, the (directionally) intersecting beams are further positively controlled both (i) duration, and (ii) time relationship (alternatively called time sequence, or phase).
When the radiation illumination of the volume of radiation-sensitive medium is controlled so as to be in the form of time-sequenced radiation pulsesxe2x80x94as it is in the 2-P 4-D memory of the present inventionxe2x80x94undesired changes outside the addressed domains of the volume will, for all practical purposes, become physically impossible. Accordingly, the 2-P 4-D memory will have a high figure of merit for selectivity, and considerable resistance to cumulative contamination during repetitive use.
Continuing with the write operation, if the radiation-sensitive medium at the addressed domains is already in its second state then it is unchanged by radiation of this first energy level. The radiation-sensitive medium is insensitive to the energy of either write radiation pulse taken alone, and nowhere within the entire 3-D volume is the radiation-sensitive medium changed in the slightest by either write radiation pulse taken individually.
Each domain within the 3-D volume of radiation-sensitive medium is also read by process of two-photon interaction. A domain is so read upon such times as two radiation pulsesxe2x80x94one of which radiation pulses may also be a pulse otherwise used for writing and which two pulses taken together have a joint energy that aggregates a predetermined second energy level less than the first energy levelxe2x80x94again temporally and spatially intersect in the domain, interacting therewith. The radiation-sensitive medium is totally insensitive to radiation of this aggregate second energy level, as well as to the energy of either beam taken individually, to change in the slightest, let alone to change state. Accordingly, reading is non-destructive.
Considering the previous paragraph, one way of describing the response of the 2-P 4-D radiation memory during reading is to say that it is selectively transparent. Neither radiation pulse can xe2x80x9cseexe2x80x9d anything within the volume of radiation-sensitive medium save that it temporally and spatially intersects the other pulse, and then only to the time extent and over the spatial interval where such intersection satisfies the quantum-mechanical equations of two-photon interaction. Both pulses xe2x80x9cseexe2x80x9d the same thing (if the molecules of the radiation-sensitive medium are randomly aligned, as is normal) at their locations of intersection, and each is modified in the same way.
Note that in the two-photon interaction each and both radiation pulses are not modified in accordance with their individual characteristics (such as frequency, and energy), but are modified identically accordance with the two-photon interaction. Both read radiation pulses accordingly xe2x80x9cseexe2x80x9d the same thing, and both are commonly detected so as to permit real-time redundant checking of the correct operation of the 2-P 4-D radiation memory.
Remarkably for a radiation memory that already has two (2) separate and redundant read radiation outputs (each of which is independently detectable in an associated array of photodetectors or the like), there is yet further radiation output from the memory during reading. The two stable indices of refraction selectively assumable by the radiation-sensitive medium represent, in the preferred spirobenzopyran radiation-sensitive medium, two different isomeric molecular forms of this spirobenzopyran medium. During reading each intersection domain will selectively fluoresce dependent upon its pre-existing isomeric molecular form.
This incoherent fluorescence occurs at a separate wavelength and frequency from each of the selectively transmitted read light pulses. It is also detectable by photodetectors or the like. Although detectable at any angle to the volume, the intersection domains will be unambiguously resolved only along an axis of illumination. The fluorescent light emissions may be split out from the coherent, illuminating, read radiation pulses also transmitted along these illumination axis by beamsplitters and like devices. Note that, because the fluorescent light emissions travel in all directions (unlike the directional read radiation pulses), such beamsplitters can be located in the path(s) of either of both read radiation pulses in locations before the volume of radiation-sensitive medium. Because there is little problem with laser production of read radiation pulses sufficiently intense so as to overcome any losses in a beamsplitter, there is little, or no, problem to redundantly detecting the fluorescence along one or both illumination axis, providing yet another redundant detection of the data stored in the intersection domains.
Although unnecessary for operation as a memory, and although projected to be uncommon during normal usage of the 2-P 4-D radiation memory, each domain within the 3-D volume of radiation-sensitive medium can still further be erased (changed from its second to its first state). Erasure is again by process of two-photon interaction, particularly two-photon absorption. A domain is erased upon such times as two radiation pulsesxe2x80x94one of which radiation pulses may also be a pulse otherwise used for writing and/or reading and which two radiation pulses taken together have a joint energy that aggregates a predetermined third energy level that is greater than the first (and second) energy levelxe2x80x94temporally and spatially intersect within the domain, interacting therewith by two-photon absorption.
The radiation-sensitive medium within each intersection domain is responsive to radiation of this third energy level to change from its second to its first state. If the radiation-sensitive medium is already in its second state then it is unchanged by radiation of the third energy level. The radiation-sensitive medium is insensitive to the energy of either illuminating erase radiation pulse taken alone, and is nowhere within the entire 3-D volume is the radiation-sensitive medium changed in the slightest by either erase radiation pulse taken individually.
The radiation memory of the present invention is deservingly called four-dimensional, or xe2x80x9c4-Dxe2x80x9d for at least two reasons.
First, and as previously explained, fluorescence from the intersection domains can be detected, preferrably along one or both illumination axis, to provide yet another redundant detection of the data that is stored within the intersection domains. The fluorescent light output is at a different frequency and wavelength from either of the read radiation pulses (that are both selectively refracted in their transmission through the volume of radiation-sensitive medium by the pre-existing indices of refraction stored in the intersection domains). Because each frequency of light output is a separate dimension, the radiation memory of the present invention is xe2x80x9c4-Dxe2x80x9d.
Second, the (i) three-dimensional (3-D) volume of a medium that is sensitive to radiation in its absorption band to undergo an anomalous change in index of refraction, and (ii) the manner of radiatively defining, changing and detecting selected domains within the 3-D volume of radiation-sensitive medium by process of two-photon (2-P) absorption, together also constitute a fourth dimension (4-D) to a standard 2-P 3-D radiation memory. Together they serve to make a radiation memory four dimensional (4-D) in a way that the inducement of selective changes in isomeric molecular form by process of two-photon absorption that is taught within the companion predecessor patent applications do not.
To understand why this is so, and the second reason why the present radiation memory is deservedly called four-dimensional (4-D) rather than three-dimensional (3-D), consider the following. Firstxe2x80x94and although any radiation pulse or beam will be somewhat affected by passage through a great length of radiation sensitive mediumxe2x80x94in accordance with the present invention a single radiation pulse is substantially unaffected by any and all domains of varying refractive index through which it passes so long as it does not undergo interaction with any domain or domains by process of two-photon absorption. Indeed, the single radiation pulse is likely totally unaffected within measurement limits during its passage through short, fractional meter, lengths of a radiation sensitive medium.
Accordingly, if the dimension of a three-dimensional (3-D) volume of radiation-sensitive medium having domains that exhibit varying indices of refraction is kept quite smallxe2x80x94say on the order of one centimeter (1 cm.)xe2x80x94then a single radiation pulse (or beam) of less than threshold energy will pass through the medium substantially totally unaffected in any way, and will incur refraction if, when and wherexe2x80x94and only if, when and wherexe2x80x94it both spatially and temporally intersects a second radiation beam of appropriate energy. One way of regarding this phenomena is to consider that the radiation-sensitive medium is transparent to the single radiation pulse or beam save only where, and when, it is selectively rendered non-transparent by a temporally and spatially intersecting pulse or beam (of appropriate energy).
Consider the involvement of time. A radiation pulsexe2x80x94whether used for writing or for reading or for erasingxe2x80x94interacts with the radiation-sensitive medium only at the particular location(s) where it is both spatially and also temporally coincidence with an intersecting pulse. The two pulses affect, and are most substantially affected by, the radiation-sensitive medium not only at the location(s) of their spatial, but also of their temporal, intersection. This means that both radiation pulses both modify, and are themselves modified, in a temporal (time), or fourth, dimensionxe2x80x94as well as in the three spatial dimensions.
The 2-P 4-D radiation memory of the present invention makes use of its temporal, or time, dimensionxe2x80x94which is why it is so named xe2x80x9c4-Dxe2x80x9dxe2x80x94in the addressing of domains within the 3-D volume of radiation-sensitive medium.
Consider reading. Each of two mutually intersecting radiation pulses exiting the volume of radiation-sensitive medium will bear the record of the refractive index of only the domains in which each pulse has intersected the other during its passage. Each pulse passes through the volume of radiation-sensitive media at near light speed, typically some substantial fraction of 3xc3x97108 meters per second. During the course of its passage each pulse intersects the other in sharply defined and located regions called domains. The volume (size) of the domains is set by (i) the speed of the pulses in the radiation-sensitive medium, and (ii) and the quantum mechanical requirement that the two pulses must be spatially and temporally coincident for a sufficient time and space so as to interact with the radiation-sensitive medium.
Curiously, and beneficially, the duration of a pulse need not be so short as the time it takes to traverse a domain that the pulse (and its companion pulse) serve to define. Consider that if each radiation pulse is traveling at some substantial portion of 3xc3x97108 meters/second, and if the volume of radiation-sensitive medium is on the order of 1 cm3, and if this volume is divided into 2xc3x97103 by 2xc3x97103 by 2xc3x97103 (i.e., 8xc3x97109) domains, then, if half the distance in each of three co-ordinate directions is devoted to domains (i.e., one-eighth the volume), then the dimension of each domain will be about 2.5xc3x9710xe2x88x927 meters and each beam will traverse this distance in about 8 femtoseconds (8xc3x9710xe2x88x9215 seconds). Laser pulses this short can be generated, but only with difficulty. Luckily, however, the quantum-mechanical equations of two-photon absorption require that each of two pulse should be considerably longer than 8 femtoseconds if it is to react by process of two-photon absorption over a distance of 2.5xc3x9710xe2x88x927 meters. In fact, each radiation pulse has a quite manageable length of about 10 picoseconds (10xc3x9710xe2x88x9212 seconds). Each intersection domain has a dimension of about (2.5xc3x9710xe2x88x925 meters)3, and 2xc3x97103 by 2xc3x97103 by 2xc3x97103 such domains (i.e., 8xc3x97109 total domains) fit within a three-dimensional volume of one cubic centimeter (1 cm3) with as much spacing in any co-ordinate direction between adjacent domains as the domains extend in that direction. (A xe2x80x9csafetyxe2x80x9d margin this great, or 100% margins, is not required, but the radiation-sensitive photochromic medium in its polymer matrix is exceedingly inexpensive, and there is little need to tightly pack the domains.)
As a final step to selecting one multiplicity of domains to be read, written, or erased during any one cycle, out of the very great number of domains that are within the entire volume, one of the pulses is variably delayed relative to the other pulse. If the radiation pulses intersect at other than 180xc2x0 (i.e., counter-propagating), and, for example, if the radiation pulses intersect at 90xc2x0 (i.e., perpendicular) then one pulse must be so variably delayed in each of various regions of its planar wavefront to a different degree.
The nature of this requirement, and the manner of so doing, is a bit challenging, and will be taught in conjunction with FIG. 4. However, three things are simple to understand.
First, the reason whyxe2x80x94for pulses that are not counter-propagatingxe2x80x94xe2x80x9cone pulse must be so variably delayed in each of various regions of its planar wavefront to a different degreexe2x80x9d is simply this: each straight line path through the 3-D volume of radiation sensitive medium from the source of each pulse to the detection of each pulse must intersect the other pulse (in space and it time) at one only domain. In other words, a pulse traveling along a single line carries the information to change but a single intersection domain, and in its further path beyond the intersection domain, carries the information of but this one domain. Alas, the manner of controlling the delay, or phase, of various regions of the pulse wavefront required to realize this straightforward requirement is conceptually difficult.
Second, even though the selective control of the delay, or phase, of various regions of one pulse wavefront in order to realize non-redundancy in the paths of both pulses (when intersecting at other than 180xc2x0) is sophisticated, the mechanism for putting the required time, or phase, delay into each pulse wavefront region is well known. A multiplicity of different phase delays may be applied to a corresponding multiplicity of regions of the pulse wavefront by means no more complex than channeling the light of each region through an associated electro-optic or acousto-optic modulator.
Third, the entire requirement that the different regions of a pulse wavefront (as opposed to the entire pulse) should be variably delayed is completely obviated when the pulses are intersecting at 180xc2x0, or counter-propagating. Accordingly, for its simplicity and elegance, a intersection of counter-propagating pulses is a preferred embodiment of the invention. The regrettable reason why the another, unpreferred, embodiment is taught first in this specification is that domain selection in the counter-propagating wavefront 2-P 4-D memory transpires almost entirely in the time domain, and would better be illustrated by a motion picture or videotape than the static drawings of a patent application.
These and other aspects and attributes of the present invention will become increasingly clear upon reference to the following drawings and accompanying specification.